In the field of medical technology and diagnostics, a large number of devices and methods for detecting at least one analyte in a body fluid are known. The methods and devices may be used for detecting at least one analyte present in a body tissue or a body fluid, in particular one or more analytes such as glucose, lactate, triglycerides, cholesterol or other analytes, typically metabolites, in body fluids such as blood, typically whole blood, plasma, serum, urine, saliva, interstitial fluid or other body fluids. Further devices are known for measuring activating times, e.g., a thrombin activation time measurement for coagulation monitoring. Without restricting the scope of the present disclosure, reference is made to the detection of glucose as an exemplary and typical analyte in the following.
The determination of blood glucose concentration as well as a corresponding medication is an essential part of daily routine for many diabetics. In order to increase convenience and in order to avoid restricting the daily routine by more than a tolerable degree, portable devices and test elements are known in the art, such as for measuring blood glucose concentration during work, leisure or other activities away from home. A large number of test devices and test systems are known that are based on the use of test elements in the form of test strips. Applications are known, in which a multiplicity of test strips is provided by a magazine, wherein a test strip from the magazine automatically may be provided to the testing device. Other applications, however, are known in which single test strips are used, which are inserted into the testing device manually by a user. For applying the sample to the test element, typical test elements provide at least one sample application site, such as a capillary opening in capillary test elements. Alternatively to home care applications, such test elements may be used in professional diagnostics, such as in hospital applications.
In many cases, for detecting the analyte, test elements are used, such as test strips, which comprise one or more test fields having one or more test chemistries. The test chemistries are adapted to change one or more detectable properties in the presence of the analyte to be detected. Thus, electrochemically detectable properties of the test chemistry and/or optically detectable properties of the test chemistry may be changed due to the influence of the presence of the analyte. For test chemistries that may be applied in the present disclosure, reference may be made to J. Hones et al.: Diabetes Technology and Therapeutics, Vol. 10, Supplement 1, 2008, S-10 to S-26.
In general, the detection of the at least one analyte can be performed by using an electrochemical test element. Herein, the test element typically comprises at least two separate electrodes connected to a suitable electronic circuit. Usually, at least one electrode, often designated as working electrode, is employed for detecting the analyte. For this purpose, the working electrode is usually covered with an electrochemical transducer configured for converting parameters with respect to the analyte into a measurable property of the electrode, in particular into an electrical current or an electrical potential. Typically, the working electrode comprises at least one detector reagent, in particular at least one enzyme, such as glucose oxidase (GOD), adapted to perform an oxidation reaction and/or a reduction reaction with the analyte. In case the detection reaction comprises an oxidation reaction at the working electrode, the counter electrode typically provides a reduction reaction in order to close the electric circuit through a measuring cell of the test element. For this purpose, the second electrode is designed in a manner that a maximum current detectable at the working electrode may pass the second electrode, thereby supporting a sufficient electrode reaction. From an application of disposable test elements for detecting an analyte concentration or a physiological activation time in a sample of a body fluid two distinct test strip configurations are known.
In a first known configuration, a test chemistry is used for covering the at least two electrodes in the test element. Herein, the two electrodes comprise the same material being selected from a noble metal, typically silver, or a carbon material. In this configuration, the test chemistry is adapted to support the analytical detection reaction at the working electrode and, at the same time, the electrode reaction at the counter electrode. As a result, a current can pass through the electrochemical test element. For purposes of producing the test element, the test chemistry is, generally, applied to the electrodes that are arranged in a co-planar manner and, subsequently, dried. During application of the test element, the test chemistry is then dissolved by the liquid sample comprising the body fluid, by which process the two electrodes become electrically connected. As described above, the electrochemical transducer is adapted to support the detection reaction. However, not all possible electrochemical transducers may simultaneously support the reaction at the counter electrode. Furthermore, particularly due to interfering reactions or insufficient reagent stability, some test chemistries cannot be combined with compounds that would otherwise be configured for supporting the counter electrode reaction.
Therefore, a second known configuration comprising the separated Ag/AgCl electrode as the counter electrode is, generally, employed with disposable test strip bio sensors. Herein, the at least two electrodes are located in a separated manner, covered with different kinds of reagents, and only connected by the sample comprising the body fluid, which works as a liquid electrolyte. In this configuration, a silver-silver chloride electrode (Ag/AgCl electrode) is used frequently. The Ag/AgCl electrode supports an anodic reactionAg→Ag++e− andAg++Cl−→AgCl,wherein, as a result of a precipitation of silver ions Ag+ with chloride ions Cl−, a coverage comprising silver chloride AgCl is obtained, or a cathodic reactionAgCl→Ag++Cl− andAg++e−→Ag,wherein silver ions Ag+ are produced by dissolving the silver chloride AgCl, wherein the silver ions subsequently return to silver atoms on the negatively charged silver layer of the counter electrode. Accordingly, the electrical potential for this electrode reaction only depends on a chloride concentration of the electrolyte, which is for a blood sample quite constant. This type of electrode reaction that comprises a precipitation and a re-dissolving step provides a rather constant electrode potential that is, further, largely independent from the electrode current that may have a potential to depolarize the electrode.
Herein, the reagent layer on the working electrode may comprise an enzyme with a redox active enzyme co-factor to support a specific oxidation of the analyte in the body fluid. The reagent layer may comprise further a redox cycle providing substance, which may act as an electron acceptor. The redox cycle providing substance or redox mediator may react with the enzyme co-factor and may transport electrons taken from the enzyme co-factor to the electrode surface by diffusion. At the electrode surface, the redox mediator may be oxidized and the transferred electrons may be detected as a current, wherein the current may be proportional to a concentration of the analyte in the body fluid. Examples for this embodiment may be found in US 2003/0146113 A1 or US 2005/0123441 A1. As a further example, detection reagent test strips commercially available under the trade name COAGUCHEK XS test (Roche Diagnostics) for measuring a pro-thrombin activation time, comprise an artificial peptide substrate, wherein the protease thrombin may specifically cut off a linked redox tag. By applying a suitable voltage, the cleavage of the redox tag then can be detected by a resulting current. Herein, an Ag/AgCl electrode is used as the counter electrode.
However, known manufacturing processes for the Ag/AgCl electrode are associated with a number of disadvantages. Usually, a silver chloride material, such as in form of an ink or a paste, is coated or printed upon conductive traces in a manner that it may not react, such as by corrosion, with the material of the conductive traces, for which, in particular, a noble metal, including silver, or carbon is used. Unfavorably, silver chloride inks or pastes are rather expensive and the corresponding manufacturing process for producing the Ag/AgCl electrode is complex, in particular due to the steps involving coating and drying. Further, the silver chloride coatings exhibit a rough surface, thus, making it difficult to laminate the test strips layers together such that the laminated test strips exhibit sufficient stability.
Furthermore, silver chloride inks or pastes are electrically conductive. Therefore, the resulting electrode structures may be short cut after coating as a stripe over the structured co-planar electrodes. To avoid this disadvantage, the strips may be coated, such as by a reel-to-reel process, at a position where only one conductive pathway stays in contact with the silver chloride paste or ink. This, however, can only be performed at the dosing side of the test strip. For example, in case of the above-mentioned COAGUCHEK XS test strip (Roche Diagnostics), in which a long sample capillary is used in order to be able to move the sample to a heater position located within the meter, the Ag/AgCl counter electrode, thus, remains far away from the working electrode and, particularly, outside a thermostatically controlled test zone. In order to place the Ag/AgCl electrode at a different position, the paste or ink must be limited to the electrode surface so that a pure reel-to-reel process will not be applicable.
Alternatively, a direct structuring of a layer of the silver chloride material in order to form at least one surface of the Ag/AgCl electrode, conductive pathways, and contact pads is also feasible. Depending on a nature of the selected structuring process, this kind of procedure which involves a local printing process could significantly increase the cost of the production and, concurrently, reduce production rate and production robustness.
A further known process for producing a silver chloride layer on a silver surface comprises an anodic polarization of a silver coated polymer foil in an electrolyte comprising chloride ions. This process, however, requires an electrolyte bath and a corresponding electrical contact during the production process. As a result, the typical reel-to-reel process, which allows producing test strips over a length of more than 500 m, is again not applicable here.
WO 2003/076648 A1 discloses such a manufacturing process for an Ag/AgCl electrode, wherein, prior to coating the silver electrode with a polymer film in order to immobilize a redox mediator and an enzyme, an anodic current is conducted through the silver electrode which is placed in a solution comprising chloride ions, by which step a thin silver chloride layer is obtained on a surface of the Ag electrode.
US 2009/0294306 A1 and US 2009/0298104 A1 each disclose a method for an in-situ renewal of the AgCl layer of an Ag/AgCl reference electrode, wherein the Ag/AgCl reference electrode has been produced before by a method according to the state of the art, such as the method described in US 2006/0016700 A1. Herein, a level of silver chloride on the Ag/AgCl reference electrode of an electrochemical sensor, which is subcutaneously implanted in a patient, is replenished by applying a brief electrical potential across the reference electrode and another electrode for a period of time being sufficient for converting silver to silver chloride in order to replenish the level of silver chloride present on the reference electrode in order to maintain a stable potential over the lifetime of the implanted electrochemical sensor. Accordingly, only the Ag+ ions which are initially generated in small amounts by a naturally-occurring dissolution of the AgCl layer are converted into additional silver chloride which is, subsequently, deposited onto the still existing AgCl layer of the reference electrode.
US 2002/0112969 A1 and EP 1 343 007 A1 each disclose a method for a generation of an AgCl layer, wherein Ag+ ions are generated in small amounts by a naturally-occurring dissolution of an Ag layer without application of an electrical potential. Accordingly, by using negatively charged Cl− ions being present in the body fluid the generated positively charged Ag+ ions may, thus, form a AgCl precipitation, however, only to a minor degree.
U.S. Pat. No. 6,153,069 A discloses a method for an in-situ generation of an AgCl layer on a silver electrode, wherein, in a specific embodiment, suitable reactants, such as ferricyanides, are used to initially generate the Ag+ ions which may, without application of an electrical potential, subsequently react with Cl− ions being present in the sample fluid in order to form a silver chloride (AgCl) precipitation. In a further embodiment, an Ag/AgCl electrode is initially generated by depositing a silver oxide layer through reactive sputtering onto an Ag film. During subsequent testing, the silver oxide layer is, without application of an electrical potential, converted in-situ into silver chloride when the test element contacts the body fluid comprising chloride ions.